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SMA Connectors


#1

I have never been a fan of BNC connectors, their ability to wag about and invisibly break the centre conductor at the most opportune moment is a big drawback. On HF the PL259 is our workhorse but it is heavy, electrically poor and not waterproof. The N type is electrically good, waterproof but even heavier. There is a connector, the TNC that combines the qualities of the N type and BNC but they are scarce and expensive.

In the interests of reliability and saving weight I have been trying SMA connectors for the past few months. These connectors are made of gold flashed stainless steel (no latinum I am afraid) and have excellent RF performance up to many GHz. They are readily available on the web at reasonable prices. If carefully assembled, with a little silicone grease, and heatshrink sleeve, they are also splash proof. In addition, converters from N Type and SO238 sockets are also available.

I use an FT 857 on most of my activations at 80Watts RF power out. Reading the specs, 100W should be considered the maximum power throughput. I have experienced no problems using a feeder with SMA connectors at both ends.

In my case I use an SMA male right angle plug at the rig (the feeder cable does not get bent when standing the rig on it’s end) and a bulkhead line socket to connect to the HF balun or VHF /UHF aerial. On the balun I fit a 4 bolt, flange mount, male, captive nut socket. I only carry one feeder cable.

Crimp SMA connectors are available to suit RG58 and RG174 cable in a variety of formats but assembly is a real art form, requiring measurements to the mm and excellent soldering skills together with a crimping tool.

I test all my feeders with a 500 Volt Megger, to confirm the conductivity and insulation. The plugs withstand this test.

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Regards
Snowbound David


#2

Hi David,

I like SMA connectors, and have started using them on 23cm. My only gripe is how fiddly they are to disconnect with gloves and or numb fingers. A spanner helps, but that’s something else to drop etc.
I’ve wondered about moulding on a grip ring using Sugru or similar, but it would be difficult to make it look nice!
https://sugru.com/about?gclid=EAIaIQobChMIuKH444TJ2QIVlhobCh25tg7rEAAYASABEgKlg_D_BwE

The problem with BNCs, I think, is that it is possible to rotate the body of the connector whilst it is plugged in.
Unfortunately, the centre pin is being gripped by the socket petals, and so resists rotation, putting a torsional stress on the centre conductor. It doesn’t take many repeated twists for the strands to become fatigued and break.
This is just my theory, but I have tested it by fitting a pin to the end of a piece of RG58 centre, plugging it into a BNC socket, and then gently twisting it one way and then the other by a few degrees repeatedly. It seems to stand up!

My solution is to put the kit in position, and then connect up - trying not to disturb the cable lay during the activation.
When I’m using a hand held with an external antenna, I loop the coax through the belt clip, and then curve it up and around to the top of the handie, so that again, the connector and cable do not move in relation to the rig during operation.

Regards
Adrian - picturesque dusting of snow in sunshine here, at present.


#3

I imagine someone with a 3D printer could come up with something. I find them way too fiddly to fit myself.


#4

Currently 20cm snow with 10cm having fallen in last 2.75hrs. Sarah’s car in the foreground was clear of snow when it arrived home at 2.00pm. Now 4.55pm.

Anyway, SMA…

5/16th AF is what you need. If you get a combi open/ring, grind a slot into the ring end so you can slip the cable through the slot and use the ring for better “traction”. when fastening both ends. But they are fiddly and not for cold WX and fingers. Worse if you have low loss cable like RF-400U with an SMA on the as just getting the connector mate is tiresome. It’s the same diameter as RG-213 but so much less loss.

I have a spanner in the 13cms gear. It has a 1m length of yellow 2mm cord attached so I can find in the heather/grass when dropped.

I always try to ensure the nut bit gets turned and the pin doesn’t. Most SMA antennas will spin the pin in the socket as you fasten them which wears the socket fingers a bit each time it is exercised.

I bought several male to female SMA adapters to place on the connectors of my transverter. They will be frequently made and removed and I didn’t want to wear them. Replacing them is not too hard but for £3, I got 2 disposal items.


#5

Hi David

I use SMA connectors for 23 and 13cm with short lengths of 50 ohm RG402 or RG58 Cell Foil, the latter for 23cm only.

13cm folded dipole

200 mm length of RG58 Cell Foil feedline for a 23cm Bi-Quad

20180206_045506

Andrew VK1AD


#6

If you search for “SMA Finger Wrench” you’ll find something like this:

Using a spanner is a terrible sin! Use the correct torque wrench, at least until you have learnt to feel the appropriate torque.


#7

It’s only a sin on those Radiall or Suhner versions rated to a bit beyond 18GHz. The £1.50 Chinese “gold-finish” ones are expendable. :slight_smile:


#8

Ah, but the trouble is it’s not immediately obvious when they’re going flakey, which the cheap ones will do quite readily.

A more grievous sin is of course abusing ‘K’ connectors. These mate with SMA but work to 40 GHz. :slight_smile:

And actually ebay tat doesn’t save you that much over proper connectors from a proper supplier. A decent crimp plug’s about 4 or 5 quid IIRC.


#9

In mitigation, I would only use a spanner to undo the connector, never to tighten it…

The “finger wrench” looks good, thanks.


#10

But I haven’t got a crimp tool…yet. having shifted a load of unused gear on ebay I can actually afford a proper one for RG316/RG174/RG58 now.


#11

Consider using push-on 'till click, pull off SMB’s, the SMA’s cousin, instead.
No need to have extra room for spanners, or gloved fingers.
image

Ken


#12

Hello,

I am using RCA connectors for HF using QRP levels in combination with RG-174 in my EFHW antenna and transformer.
kk

They work great, very cheap and easy to solder / replace.

They probably have losses but I guess they are acceptable for the small run of coax I use… And I don’t need any tool to assemble/ disassemble .

73 de Ignacio


#13

Hi David,

no sure what you mean by the PL-259 being electrically poor. It’s been proven in multiple independent tests that the losses are negligible in HF/VHF (0.1dB? being a typical value). It also comes with a mostly overlooked but very important advantage, that it has a very wide surface of contact for the center pin, which is essential when operating in the field as it avoids imperfect contacts even when dirt gets in or cables are moved. It’s even easy to refit with minimal tools & knowledge.

BNC or SMA are lab-class connectors, with lower losses but inferior when it comes to robustness and ease of use.

Cheers,
Razvan.


#14

Hi Razvan PL259 electrically poor…

  1. Non constant impedance, yes that matters not at HF for short transitions. but at VHF and above it is significant. I activate on 5 MHz to 432MHz. with the same feeder.

  2. I have had occasions where the center conductor has not connect with the socket, being in the center of the void between the tangs of the socket conductor. This condition is easy to achieve having inserted a plug with a center pin that is overloaded with solder.

  3. Some PL259s are fitted with glands, these are superior but I have seen many example of RG213 cable fitted to standard PL259s where the user has made a “dogs breakfast” of fitting the screen. Hence the need to carefully test the plug/cable interface.

  4. I have never seen a satisfactory screen connection on a standard PL259 fitted with reducer when used with RG58.

5 PL259 plugs designed for RG173 do exist but are difficult to source.

I agree that BNC’s should be confined to the shack or lab. However have thus far found SMA’s to be robust, light, splash proof, cheap and versatile, but a challenge to assemble and a little fiddly to unscrew on the hill.

Have fun

David


#15

One of the problems you may not have hit yet is that most SMA connectors are only rated for a quite low number of connect/disconnect cycles. I’ve seen figures as low as 250 quoted.

Colin


#16

Three cheers for RCAs!

I work with SMAs in my day job, I wouldn’t want to use them out in the field - I think they’re definitely in the ‘fit and forget’ category. I often have to replace SMA sockets that have become unreliable causing return loss test failures in the equipment I service.

73, Colin


#17

One reason I use SMAs is so that I can use the same patch leads for both my HF and VHF gear. They also link easily with MCX (via an adaptor) so I can plug the Sark 110 in without yet another patch lead. But they are certainly not the best in cold weather. I like the idea of SMBs, might give them a go.
de OE6FEG / M0FEU
Matt